Hydraulic actuators, in which a piston fits within a cylinder and is forced to move along the cylinder by pressure differences in a fluid on either side of the piston, are used in a variety of applications. Hydraulic actuators are commonly used in the control of machines and structures that are subject to large forces. For example, these actuators are used on rotary and fixed-wing aircraft to counter and control the large forces that develop during the flight and landing of the aircraft. Hydraulic actuators may be used on such aircraft to provide position control of equipment such as nose wheel landing gear, main landing gear, speed brake control surfaces, and flap control surfaces.
For some applications such as those above, it may be desirable under certain conditions to lock the position of the piston of the hydraulic actuator at a particular location relative to the cylinder. Often one positional extreme or the other of the piston movement or xe2x80x9cstrokexe2x80x9d is selected. The positional extremes of the piston are sometimes referred to as the xe2x80x9cextend positionxe2x80x9d and the xe2x80x9cretract position.xe2x80x9d Such hydraulic actuators with locking capabilities are commonly referred to as locking hydraulic actuators or locking actuators.
Different locking mechanisms have been used to lock hydraulic actuators. Hydraulic actuators may be locked through hydraulic locking, which can result when the hydraulic fluid is prevented from flowing within the hydraulic circuit of the hydraulic actuator, thus preventing movement of the piston within the cylinder. This type of locking relies on the pressurization of the actuator and may not be reliable when pressure in the actuator is lost, which can occur when a leak occurs in the hydraulic circuit of the actuator, when the hydraulic pump that supplies fluid to the hydraulic actuator is non-operational, or when contaminants in the hydraulic fluid block passageways or components in the hydraulic system. A piston may also be locked within the cylinder of a hydraulic actuator by mechanically interlocking parts.
Problems result from mechanically locking the hydraulic actuator. Among these problems is that such locking mechanisms are prone to unlocking from a locked position after repeated loading or heavy use, due to excessive deformation of the locking mechanism parts. Unlocking may occur when parts in the locking mechanism become deformed due to loading conditions that exceed the yield strength of the material of the locking mechanism parts. Deformation typically occurs when such hydraulic actuators experience large axial loads, particularly such loads that are cyclic in nature, i.e., that alternate between tension and compression along the longitudinal axis of the hydraulic actuator. These types of loading conditions can occur in many different situations, including for example, within a piston actuator used in landing gear of an aircraft upon landing.
Information related to attempts to address these problems can be found in U.S. Pat. No. 4,167,891 and U.S. Pat. No. 4,295,413. However, each one of these references suffers from one or more of the following disadvantages: excessive deformation of slots in main piston under axial loading of the actuator, and propensity for rotation of lock segments wider such axial loading, with resulting possibility for failure of the locking mechanism in the actuator.
For the foregoing reasons, there is a need for a locking hydraulic actuator that is able to repeatedly withstand cyclic axial loading conditions in a locked position without considerable deformation of the locking mechanism components.
The present invention is directed to a locking hydraulic actuator that satisfies this need for the capability to repeatedly withstand cyclic loading conditions in a locked position without considerable deformation of the locking mechanism components, thereby avoiding deformation-induced failure of the locking mechanism and the resulting undesired unlocking of the hydraulic actuator.
A first embodiment of the present invention includes a locking hydraulic actuator including a cylinder having an inner radial surface, a longitudinal axis, and a cylinder stop surface. A main piston slides within the cylinder and has a lock piston bore with one or more slots passing from an outer radial surface to the lock piston bore. The main piston may include a main piston head, and the main piston head may have a main piston head diameter that is larger than that of the main piston. The cylinder may be connected to a tailstock housing. A lock piston slides within the lock piston bore from a first position to a second position in either direction along the longitudinal axis of the cylinder. The lock piston has a first section with a first diameter, and a second section with a second diameter greater than the first diameter. The lock piston may an intermediary section with a diameter that varies from the first diameter to the second diameter. An elastic coupler, which may be a spring, connects the main piston to the lock piston, and the spring tends to keep the lock piston preloaded in one direction. One or more lock segments are included. The lock segments slide within the slots and each lock segment has a proximal straight taper and a distal straight taper and a cylinder-abutting surface. The straight tapers may include an intersection of two flat faces of the lock segment. Each lock segment also may have an outer radial surface and an inner radial surface, a first and a second lateral face, and a proximal face and a distal face. The lock segments are radially moveable from a locked position where each of the cylinder-abutting faces contacts the cylinder stop surface and in which position the main piston is immovable along the actuator longitudinal axis to an unlocked position in which the main piston is movable along the longitudinal axis of the actuator.
A second embodiment of the present invention includes an improvement for a locking hydraulic actuator of the type in which a main piston is slidably disposed within a cylinder, and wherein the main piston has a lock piston slidably disposed within the main piston. This type of hydraulic actuator has one or more lock segments that are radially slidingly disposed within slots in the main piston and radially moveable from a locked position to an unlocked position. The improvement includes a proximal straight taper disposed on a proximal face of each lock segment, and a distal straight taper disposed on a distal face of each lock segment. The proximal straight taper transmits and distributes stresses from axial loads developed in the locking hydraulic actuator across the entire width of the lock segment to the proximal face of the slot. Similarly, the distal straight taper transmits and distributes stresses of the axial loads developed in the locking hydraulic actuator across the entire width of the lock segment to said distal face of each slot. The improvement may include the lock piston having a portion that contacts the one or more lock segments with a constant diameter while the lock segments are in a radially extended position in which the actuator is locked. The main piston may include a main piston head, and the main piston head may have a main piston head diameter that is larger than that of the main piston.
A third embodiment of the present invention includes a locking mechanism for a piston and cylinder assembly. The locking mechanism may include a lock piston slidingly disposed within the piston of the piston and cylinder assembly. The piston may include a piston head, and the piston head may have a piston head diameter that is larger than that of the piston. The lock piston has an intermediary section between a first section with a first diameter and a second section with a second diameter greater than the first diameter. Included are one or more lock segments that are placed in one or more corresponding slots disposed through the piston from an outer radial surface to an inner radial surface. Each of the one or more lock segments has a proximal face having a proximal straight taper, a distal face having a distal straight taper, and a width. Each of the one or more slots has a proximal face and a distal face. The lock piston is movable from a locked position to an unlocked position.
In the locked position, the second section of the lock piston rests radially inward of the inner radial faces of the lock segments. In the unlocked position, the first section rests radially inward of the inner radial faces of the lock segments. When the lock segments are in a locked position under axial loading conditions, the proximal straight taper transmits and distributes stresses arising from the axial loads developed in the piston and cylinder assembly across the width of the lock segment to the proximal face of the slot. Under the same conditions, the distal straight taper transmits and distributes stresses of across the width of the lock segment to the distal face of the slot. The one or more lock segments, in response to movement of the lock piston, are moveable radially from a locked position in which the piston is locked within the cylinder to an unlocked position in which the piston is moveable within cylinder. The lock piston second section may have a constant diameter.
The various embodiments of the present invention may also include a lock position indicator mechanism. The present invention may be used with different types of hydraulic control systems including but not limited to three-way and four-way electrohydraulic servo valves of the closed-center (overlap), open-center (under lap), or critical-center types (zero lap) and two-way and three-way solenoid valves. The present invention may also include one or more single-rod actuators or double-rod actuators and varying number of lock segments. In preferred embodiments, the portion of the main piston that includes the slots may have a diameter that is between five and ten thousands of an inch, i.e., mils, less than the diameter of the main piston head in the cylinder. Maximizing the diameter of the main piston in this manner increases the surface area of the slot faces and the area over which forces can be distributed to the lock segments. Also in preferred embodiments, the lock segments and the slots may have a clearance that is between one-half and three mils. By minimizing the clearance with the lock segments in this manner, lock segment rotation is minimized.